There have developed over the years hermetic helical screw rotary compressor units particularly useful in the refrigeration and air conditioning field characterized by an outer hermetic casing which houses, in an axial array, an inner casing bearing intermeshed helical screw rotors and an electric motor at one end of the intermeshed rotors. The electric motor rotor is shaft connected directly to one of the intermeshed helical screw rotors to drive the other of said helical screw rotors. The hermetic outer casing may additionally include primary, secondary and even tertiary oil separators to effect discharge of a substantially oil free refrigerant in vapor form from the outer hermetic casing. Oil for lubricating and sealing purposes, as well as for inducing some cooling action to the compression process, may be injected under pressure at various points along the inner casing housing the intermeshed helical screw rotors, as for instance at an injection port opening to the intermeshed helical screw threads defining a closed thread or compression chamber, and at bearings for the motor rotor if separate bearings are provided for that component.
In such hermetic helical screw rotary compressor systems or units, particularly of hermetic design, in order to vary the capacity of the compressor from full load to full unload, there has developed the use of a slide valve which is axially shiftable relative to the compressor inner casing and covers a return passage returning a portion of the working fluid, prior to compression, to the inlet or suction port of the machine, i.e. bypassing the compression process depending upon the system load requirements. In order to shift the slide valve, there is conventionally provided a hydraulic cylinder in which the cylinder sealably houses a slidable piston which is displaceable in a direction to load the compressor and which is directly fixed by a piston rod to the slide valve. Spring means or other means tend to bias the slide valve to full unload position, absent the application of hydraulic fluid such as oil under pressure to a chamber at one side of the piston to drive the slide valve towards full load.
One such arrangement is exemplified by U.S. Pat. No. 3,408,827 to H. Soumerai et al issued Sept. 19, 1967, and assigned to the common corporate assignee. In that refrigeration system, oil separated within the hermetic outer casing downstream from the electric drive motor or in conjunction therewith is directed to an oil sump, external of the hermetic unit, via suitable tubing and the lubricating oil at essentially compressor discharge pressure is employed as the hydraulic fluid for the slide valve operating hydraulic cylinder. Oil supplied to a closed chamber to one side of the piston within the slide valve hydraulic cylinder selectively drives the slide valve towards compressor full load position. Again, piping or tubing leads from the sump externally of the hermetic unit to the hydraulic cylinder.
Additional U.S. patents such as patents U.S. Pat. Nos. 3,738,116 to Edward S. Gazda issuing June 12, 1973, and 3,795,117 issuing Mar. 5, 1974, to Harold W. Moody Jr. et al, both assigned to the common assignee, highlight the complex tubing or piping needed for supplying hydraulic oil to the hermetic compressor unit controlled by various solenoid operated and other valves within the lines leading from the oil sump and/or oil cooler to the hermetic compressor unit. Even where the hermetic unit outer casing functions as the oil sump, as for instance in vertical axis hermetic compressor units, and when there is no external oil sump or cooler, it is conventional to feed oil for the hydraulic cylinder driving the unloading slide valve through various pipes or tubes, manifolds and the like via parallel lines to and from the compressor with suitable shutoff valves, needle valves, solenoid operated control or metering valves, all within tubing or piping external of the inner casing. Necessarily, a long length path exists from the source of the oil to the point of ultimate use, and additionally, the control valves, needle valves and the like, exterior of the hermetic unit, are subject to ambient temperature conditions. All of this adversely affects the response time and in fact the quality of the control, particularly at the time compressor operation is initiated and the refrigeration and/or air conditioning system employing the same is at "start up".
As an example of problems which exist in the field, when a hermetic helical screw rotary compressor system shuts down, the oil within the lines external of the compressor hermetic casing and leading to and from the various control valves, needle valves and the like, is subjected to significant viscosity change. Under relatively cold ambient temperature conditions, this oil which functions as the hydraulic fluid for operating the slide valve unload cylinder, becomes highly viscous with subsequent delay in slide valve operation or actual prevention of slide valve shifting due to the high viscosity of the oil initially fed to the closed cylinder chamber bearing the slide valve piston.
As may be appreciated, since the needle valves meter the flow, and thus the response of the slide valve hydraulic cylinder to a system load or unload signal, the highly viscous oil is either unable to flow through the unload or load rate needle valves or flow is substantially reduced. In the past, the compressor unit operator may mechanically adjust, i.e. open the needle valves at start up and reset the needle valves after warm up for normal operation. Such operation seriously affects the quality of the compressor operation, requires adjustments both at time of starting and subsequent thereto, and is highly inappropriate.
Additionally, another problem exists. The oil which occupies a portion of the hermetic outer casing along with the refrigerant at discharge pressure, tends to pick up refrigerant since the refrigerant is miscible in the oil. When the lubricating oil is then used as the hydraulic fluid for the unloading cylinder, the refrigerant in the oil expands when the pressure is reduced and as the oil is bled back to the suction side of the machine under compressor unloading. However, the expansion tends to create a gas volume replacing the oil in the slide valve cylinder behind the piston, so that the piston will not move, irrespective of continued pulsing, i.e. a control signal attempting to effect slide valve cylinder piston shifting towards unload direction.
It is, therefore, a primary object of the present invention to provide a modular unloading slide valve control assembly which utilizes lubricating oil under compressor discharge pressure as a source of hydraulic fluid for the slide valve unloading cylinder, wherein the oil lines for the system are substantially reduced in length, the components are in modular form, and are integrated to compressor hermetic casing so that the oil within the control valve passages is maintained at near oil sump temperature, wherein the control system components may be readily adjusted, modular portions thereof readily replaced, and wherein the system is substantially reduced in cost as compared to prior practice.